Barrage Balloons

Barrage balloons are large
balloons tethered to the ground or the deck of a ship with metal cables.
They are deployed as a defense against low-level air
attack, damaging aircraft on collision with the cables or, at the least,
making flying in the vicinity treacherous.

Barrage balloons first saw wide use in 1938, the objective being the protection of
cities and key Allied targets like ports and factories. German dive bombers
were thus forced to stay above at altitudes no less than 5,000 feet. In 1940, 1,400 balloons were deployed over London alone.